The concept of a vacant or empty niche has been controversial in ecology. It is at the basis of any discussion of whether equilibrium or nonequilibrium conditions prevail in ecological systems. A vacant niche can be defined as the possibility that in ecosystems or habitats more species can exist than are present at a particular point in time, because many possibilities are not used by potentially existing species .
1. History of the concept
Hutchinson (1957) was apparently the first who considered the possibility of vacant niches. He writes: (p.424): “The question raised by cases like this is whether the three Nilghiri Corixinae fill all the available niches……….. or whether there are really empty niches.”…….“The rapid spread of introduced species often gives evidence of empty niches, but such rapid spread in many instances has taken place in disturbed areas.”Since then, the concept “vacant niche” or “empty niche” has been used regularly in the scientific literature. Some of the many examples are        and . Further examples, some of them in great detail, are discussed in . See also Niche restriction and segregation.
2. Causes of vacant niches
Vacant niches can have several causes.One cause is radical disturbances in a habitat (biotop). For example, draughts or forest fires can destroy a flora and fauna partially or completely. However, in such cases species suitable for the habitat usually survive in the neighbourhood and colonize the vacated niches, leading to a relatively fast reestablishment of the original conditions.Further causes of vacant niches are radical and long-lasting changes in the environment, such as ice ages.Vacant niches can also be due to evolutionary contingencies: suitable species did not evolve for usually unknown reasons.For references and a more detailed discussion see .
3. Demonstration of vacant niches
Vacant niches can best be demonstrated by comparing the spatial component of niches in simple habitats. For example, Lawton and collaborators compared the insect fauna of the bracken , Pteridium aquilinum, a widely distributed species, in different habitats and geographical regions and found vastly differing numbers of insect species. They concluded that many niches remain vacant (e.g., ).Rohde and collaborators have shown that the number of ectoparasitic species on the gills of different species of marine fishes varies from 0 to about 30, even when fish of similar size and from similar habitats are compared. Assuming that the host species with the largest number of parasite species has the largest possible number of parasite species, only about 16% of all niches are occupied. However, the maximum may well be greater, since the possibility cannot be excluded that even on fish with a rich parasite fauna, more species could be accommodated (recent review in ) Using a similar way of reasoning, Walker and Valentine  estimated that 12-54% of niches for marine invertebrates are empty.The ground breaking theoretical investigations of Kauffman  and Wolfram  also suggest the existence of a vast number of vacant niches. Using different approaches, both have shown that species rarely if ever reach global adaptive optima. Rather, they get trapped in local optima from which they cannot escape, i.e., they are not perfectly adapted. As the number of potential local optima is almost infinite, the niche space is largely unsaturated and species have little opportunity for interspecific competition.The packing rules of Ritchie and Olff  can be used as a measure of the filling of niche space. They apply to savanna plants and large herbivorous mammals, but not to any of the parasite species examined so far. It seems likely that they do not apply to most animal groups. In other words, most species are not densely packed: many niches remain empty .That niche space may be nonsaturated, is also shown by introduced pest species. Such species lose almost without exception all or many of their parasites . Species that could occupy the vacant niches either do not exist or, if they exist, have not had the chance to invade these niches.Diversity of marine benthos, interrupted by some collapses and plateaus, has increased from the Cambrian to the Recent, and there is no evidence that saturation has been reached . Simulations of latitudinal gradients in species diversity using the Chowdhury ecosystem model have shown that results are closest to reality when many niches are kept empty .
4. Consequences of the nonsaturation of niche space
The view that niche space is largely or completely saturated with species is widespread. It is thought that new species are accommodated mainly by subdivision of niches occupied by previously existing species, although an increase in diversity by colonization of large empty living spaces (such as land in the geologic past) or by the formation of new bauplans also occurs. It is also recognized that many populations never completely reach a climax state (i.e., they may come close to an equilibrium but never quite reach it). However, altogether the view prevails that individuals and species are densely packed and that interspecific competition is of paramount significance. According to this view, nonequilibria are generally caused by environmental disturbances.Many recent studies (above and ) support the view that niche space is largely unsaturated, i.e. that numerous vacant niches exist. As a consequence, competition between species is not as important as usually assumed. Nonequilibria are caused not only by environmental disturbances, but are widespread because of nonsaturation of niche space. Newly evolved species are absorbed into empty niche space, that is, niches occupied by existing species do not necessarily have to shrink.
5. Relative frequency of vacant niches in various groups of animals and plants
Available evidence suggests that vacant niches are more common in some groups than in others. Using SES values (standardized effect sizes) for various groups, which can be used as approximate predictors of the filling of niche space  have shown that SES values are high for animal populations which occur in large population densities and/or are of large body size and are vagile, they are low for animal groups which occur in small population densities and/or are of small body size and have little vagility. In other words, more vacant niches can be expected for the latter.
6. Criticisms of the concept
The concept of vacant niche is not accepted by all. The reason given is that a niche is a property of a species and does therefore not exist if no species is present. In other words, the term is thought to be “illogical”. However, some authors who have contributed most to the formulation of the modern niche concept (Hutchinson, Elton) apparently saw no difficulties in using the term. If a niche is defined as the interrelationship of a species with all the biotic and abiotic factors affecting it, there is no reason not to admit the possibility of additional potential interrelationships. So, it seems logical to refer to vacant niches.Furthermore, it seems that authors most critical of the concept vacant niche really are critical of the view that niche space is largely empty and can easily absorb additional species. They instead adhere to the view that everything is much of the time in equilibrium (or at least close to it), resulting in a continual strong competition for resources. This view, indeed, is the basis of Darwinian natural selection. Many recent studies, some empirical , some theoretical, have provided support for the alternate view that nonequilibrium conditions are widespread (see above and the recent review in . In the German literature, an alternate term for vacant niches has found some acceptance. It is that of “freie ökologische Lizens” (free ecological license) . It has the disadvantage that it does not convey immediately and easily what is meant, and it indeed does not correspond exactly to the term vacant niche. The usefulness of a term should be measured on the basis of its pregnancy and easy understandability, and on how fertile it is in promoting future research. The term vacant niche appears to fulfill these requirements.
7. Vacant niches as a justification for introducing species
Pimm (1991) writes that the concept of communities containing vacant niches has been historically used to justify introduction of species (see also Herbold and Moyle 1986 ). There are many examples of species introduced into foreign habitats that have led to considerable ecological damage. We mention only the cane toad, the prickly pear and feral cats in Australia. The fact that niche space is unsaturated, does not mean that any species will be accepted into an ecosystem. After all, niche space is generally filled by newly evolving species over evolutionary time spans. Therefore, in order to avoid potentially disastrous damage, each introduction has to be justified by thorough ecological and possibly experimental studies.
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Latitude-niche width hypothesis, Competitive exclusion, Paradox of the plankton, Niche restriction and segregation, Free markets and ecology, Competition.
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